.app {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}
.box {
  width: 400px;
  height: 70px;
  overflow: hidden;
  background-color: #edd;
  li {
    height: 70px;
    width: 50px;
    float: left;
    position: relative;
    div {
      position: absolute;
      top: 0;
      left: 0;
      width: 50px;
      height: 140px;
      img {
        float: left;
        display: block;
      }
    }
  }
  span {
    display: block;
    height: 70px;
    width: 50px;
    background: url('../img/b.jpg');
    &.active {
      background: url('../img/c.jpg');
    }
  }
}
